External programs can be invoked from Java code using the exec() method of the java.lang.Runtime class. The exec() method returns an object of a subclass of the abstract class java.lang.Process. The exitValue() method can be used to observe the return value of the process. This recommendation discusses several issues resulting from the improper use of the exec() method.
Noncompliant Code Example
This noncompliant code example invokes notemaker, a hypothetical cross-platform notepad application. However, the program does not wait for the notemaker process to terminate, and throws an IllegalThreadStateException if the notemaker process has not completed when the exitValue() method is called.
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|---|---|---|
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public class Exec {
public static void main(String args[]) {
try {
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("notemaker");
int exitVal = proc.exitValue();
} catch (Throwable t) { t.printStackTrace();}
}
}
|
Noncompliant Code Example
In this noncompliant code example, the waitFor() method blocks the calling thread until the invoked process terminates. The code can be changed as shown in the modified noncompliant example below. This also has a shortcoming; the program may be blocked for a long time because of the limited buffer size used for the standard output streams on many platforms.
| Code Block | ||
|---|---|---|
| ||
public class Exec {
public static void main(String args[]) {
try {
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("notemaker");
int exitVal = proc.waitFor();
} catch (Throwable t) { t.printStackTrace();}
}
}
|
Compliant Solution (1)
An inefficient solution is to exhaust the output (or stderr) stream before beginning to wait for the process. A better option is to empty both the stderr and output streams. The code below shows this but is not the best solution since it does not process any arguments passed to the external program (notemaker) and in turn exits with an OS-specific non-zero exit code.
| Code Block | ||
|---|---|---|
| ||
public class Exec {
public static void main(String args[]) {
try {
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("notemaker");
InputStream is = proc.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
while ( (line = br.readLine()) != null)
System.out.println(line); // prints the error lines
int exitVal = proc.waitFor();
} catch (Throwable t) { t.printStackTrace(); }
}
}
|
Compliant Solution (2)
The second compliant solution spawns a command interpreter and executes the user supplied command. It uses a separate OutputStream to write the output that is read in from the external process.
| Code Block | ||
|---|---|---|
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class Exec extends Thread {
InputStream is;
String type;
OutputStream os;
Exec(InputStream is, String type) {
this(is, type, null);
}
Exec(InputStream is, String type, OutputStream redirect) {
this.is = is;
this.type = type;
this.os = redirect;
}
public void run() {
try {
PrintWriter pw = null;
if (os != null) {
pw = new PrintWriter(os);
}
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line=null;
while ( (line = br.readLine()) != null) {
if (pw != null) {
pw.println(line);
pw.flush();
}
System.out.println(type + ">" + line);
}
if (pw != null)
pw.flush();
} catch (IOException ioe) { ioe.printStackTrace(); }
}
}
public class ExecMe {
public static void main(String[] args)
{
// ... perform command argument check ...
try {
FileOutputStream fos = new FileOutputStream("c:\\output.txt");
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("notemaker");
// any error message?
Exec errorGobbler = new Exec(proc.getErrorStream(), "ERROR");
// any output?
Exec outputGobbler = new Exec(proc.getInputStream(), "OUTPUT", fos);
errorGobbler.start();
outputGobbler.start();
// any error?
int exitVal = proc.waitFor();
errorGobbler.join(); // handle condition where the
outputGobbler.join(); // process ends before the threads finish
fos.flush();
fos.close();
} catch (Throwable t) { t.printStackTrace(); }
}
}
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Risk Assessment
Misuse of the exec() method can lead to runtime exceptions and denial of service vulnerabilities.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
FIO01- J | low | probable | medium | P4 | L3 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
| Wiki Markup |
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\[[API 06|AA. Java References#API 06]\] method [exec()|http://java.sun.com/javase/6/docs/api/java/lang/Runtime.html#exec(java.lang.String)] \[[Daconta 00|AA. Java References#Daconta 00]\] \[[Daconta 03|AA. Java References#Daconta 03]\] Pitfall 1 |
FIO00-J. Canonicalize path names originating from untrusted sources 08. Input Output (FIO) MSC33-J. Prevent against SQL Injection